The continuing increase of data traffic keeps the pressure on the backbone telecommunication networks. In order to satisfy the growing bandwidth demands, more diverse and more intelligent allocation of capacity is required. Optical networking has become a key technology to accommodating rapidly expanding Internet traffic. New optical networks are expected to support the increasing network load by employing both sophisticated transmission (dense wavelength division multiplexing (WDM)) and switching (optical switches and cross-connects) technologies.
Dense wavelength division multiplexing, the transmission of multiple wavelengths over a single strand of optical fiber, has become the foundation providing the capacity and traffic separation capabilities required in the future Optical Internet. A key enabling technology of DWDM is optical wavelength multiplexing and demultiplexing, which aggregates wavelengths in the 1550 nm passband of low fiber attenuation. FIG. 1 shows a typical DWDM implementation for a point-to-point-link.
In IP networks, performance and scalability concerns prompted development of layered mechanisms providing various levels of traffic aggregation supported by DiffServ and MPLS (multi-protocol label switching) standards. In case of the optical networking, the same cost and scalability concerns translate into creation of multiple switching granularities, such as wavelengths and wavebands. The optical networking paths thus form a hierarchy in which a higher-layer path (waveband) consists of several lower layer paths (wavelengths). The potential cost benefits of wavelength aggregation into wavebands was previously demonstrated by Y. Suemura, I. Nishioka, Y. Maeno and S. Araki, Routing of Hierarchial Paths in an Optical Network, Proceedings of APCC 2001.
As illustrated in FIG. 1, the wavebands conventionally are uniform and fixed. In other words, given input wavelengths λ1-λ160, the conventional waveband groups comprise four groups of forty contiguous wavelengths, λ1-λ40, λ41-λ80, λ81-λ120,λ121-λ160. Use of uniform wavebands follows conventional wisdom in network design whereby aggregation (grooming) of communications channels is provided in a uniform manner. SONET is an example of uniform aggregation (grooming) of communications channels.
A waveband path occupies only two (input and output) ports of an optical switch in a cross-connect system. The path hierarchy reduces costs of a cross-connect system since a waveband can be switched optically as a single unit, thus reducing the number of more expensive (optical-electrical-optical) OEO ports required for processing individual wavelengths.
Cost-efficient implementation of the optical hierarchy has to be delivered by appropriately designed routing and scheduling algorithms. Routing and wavelength assignment algorithms were extensively studied in the general context of optical networking, such as in R. Ramasawami and K. Sivarajan, Optical Networks: A Practical Perspective, Morgan Kaufmann Publishers, 1998. The hierarchy of wavelengths and wavebands can be cast in several models posing new routing and scheduling challenges.
As previously discussed, the waveband hierarchical path reduces cost of a cross-connect system because a waveband path occupies only two (input and output) ports of an optical switch in a cross-connect system. However, switching exclusively in the optical domain is not practicable. Contention for the same output fiber among different wavebands cannot be resolved in the transparent (optical) part of the system. The optical core also cannot process a waveband if different wavelengths in it have to be switched into different output fibers. For these and other related tasks (such as adding a wavelength into a waveband), one or more wavebands have to be dropped to the OEO part of the optical cross-connect system. The OEO is equipped with multiplexers and demultiplexers, each of them capable to process a waveband consisting of G wavelengths. The hierarchical cross-connect system functionality is realized by hybrid optical systems consisting of a waveband (optical transparent) and a wavelength (opaque OEO) switch. The detailed architecture of the hierarchical hybrid optical cross-connect system can be based either on a single plane architecture or on multiple planes architecture.
As an example of the uniform waveband in a hybrid hierarchy, FIG. 4 shows a hierarchical hybrid optical cross-connect system with M input and output fibers. Each fiber carries N wavelengths (typical numbers are N=160 and N=40 for backbone and metro networks). Upon reaching the optical cross-connect system, all N wavelengths in each input fiber are partitioned by waveband deaggregators (denoted as WDA in the figures). WDAs can be realized using either interleavers or filters, to partition the N wavelengths into K wavebands (each consisting of G wavelengths) where N=KG. The waveband level of the optical hierarchy thus consists of wavebands each comprised of G wavelengths. The wavebands are optically switched and aggregated (using waveband aggregators, denoted by WA in the figures) into output fibers by an optical core of the hierarchical cross-connect system. The optical core may be realized by a single optical switch or by K parallel optical switches (FIG. 5, O/O1 to O/OK), each handling the same waveband from all the incoming fibers.
As shown in FIG. 6, a wavelength path input to the wavelength switch can be either directly routed to a neighbor cross-connect system (shown as flow A) or aggregated into a waveband path which is then routed to a neighbor cross-connect system via a waveband switch (shown as flow B).
The OEO port is an expensive resource depending on the technology and the transmission speed, OEO ports can be between two and five times more expensive than optical ones. Thus the design of hierarchical hybrid optical cross-connect system requires taking into account the impact of the wavelength aggregation into wavebands. Specifically, the size G of a waveband directly affects both the cost and performance of the cross-connect system. On one hand, the small number G (fewer wavelengths per waveband) creates a large number of wavebands which have to be switched by a large and expensive waveband optical core of the cross-connect system. On the other hand, the large number G (more wavelengths per waveband) increases the need for OEO conversion, as large wavebands create more wavelength conflicts, as well as wavelength aggregation and deaggregation overhead. This creates the need for a large and expensive OEO part of the cross-connect system. Conventionally, uniform wavebands had to be resolved to isolate individual wavelengths of interest. Therefore, in the example of FIG. 1, if λ12 were to be isolated for local processing (add, drop or switching), each and every wavelength of the waveband λ1-λ40 had be resolved through an OEO switch. It is clear that this conventional approach through the OEO switch is costly in terms of performing unnecessary processing for λ1-λ11 and λ13 to λ40 and the number of additional OEO ports needed to isolate the single wavelength λ12.
Cost-performance analysis of a hierarchical hybrid optical cross-connect system has been performed. It has previously been determined that the waveband size close to G=6 provides a reasonable performance (50% of the optimal one) for significant cost reduction (by the factor of 5-10). Analysis of network-level performance suggests the similar range for optimal waveband size (close to G=8). Hierarchical routing and optical wavebands can reduce the cost (measured in terms of the number of ports required to process a given traffic load in the network) by two-three times, in comparison with traditional OEO-based solutions.
The cost advantage of the optical hierarchy is based on the fact that a waveband can be switched by the optical cross-connect system as a single unit, thus reducing the number of expensive (optical-electrical-optical) OEO ports required for processing individual wavelengths. The optical paths thus form a hierarchy in which a higher-layer path (waveband) consists of several lower layer paths (wavelengths). In order to avoid expensive OEO conversion of wavelengths, the flows destined to individual output fibers should be aggregated in preconfigured wavebands, which are then switched in the optical domain. The wavebands can be created when there is sufficient number of wavelengths routed along the same path direction. Routing algorithms, optical impairment considerations and wavelength contention resolution also affect the creation of wavebands. One skilled in the art would understand the basic mechanism of assignment of wavelengths into wavebands. Details of that assignment are not provided here.